The Republic of Korea has chosen its alignment strategies and policy actions based on international dynamics and domestic constraints. South Korea’s domestic politics have had different effects on the recent administrations. Park Geun-hye had to accept the discontinuity of her foreign policy when she faced impeachment. Moon Jae-in, however, has pursued his dreams for inter-Korean relations despite systemic and domestic obstacles. In the spirit of neoclassical realism, this study identifies state-society relations and domestic institutions as key interventions in the calculus of foreign policy behavior amidst the critical influence of systemic variables. Whereas the Park administration disregarded the dynamics of domestic politics by putting state security above all else, the Moon administration has pursued a détente policy with a deep awareness of domestic politics. In a restrictive strategic environment, South Korea’s policy options are limited and the optimal choices are not necessarily the ideal ones. Foreign policy actions based on the dynamics of systemic structures and domestic politics have significant implications for Northeast Asia. State-society relations and domestic institutions have implications for the US-South Korea-Japan strategic triangle. Different strategic interests in the region place the triangle at risk in dealing with the North Korean security problem.